If you feel really nervous about shooting her topless, ask her to do a few poses with her top still on, "just to check the lighting". Take a few headshots, perhaps with her top down just enough to bare her shoulders. Talk to her while you shoot these.
You're not really checking the lighting, you are building rapport with her. By doing this before she is topless, you are both more comfortable, and you can build some trust.
Show her a shot or two, if that's your style (some photographers do, some don't). I find it helps, because she can see that you are competent at capturing the image, framing her nicely, and so on.
Only takes a few minutes, but it means she can take her top off and feel safer, and you'll be less nervous.
You can also use the time to let any marks on her skin fade (if she was wearing a bra, get her to remove it before this), so it's dual-purpose.
I don't always do this, but it helps, sometimes for my nerves, sometimes for theirs (not all models are comfortable stripping off in front of strangers... if she strips in another room, and comes out wearing a towel or coat, it's a good indication that you need to build rapport and trust!).
Unexpectedly, I have managed to get some really nice headshots from these "warm-up" sessions - bonus!